The present invention relates to apparatus for indicating the freezing of the surface of an asphalt road, paved runway, or the like. The apparatus comprises a detecting transmitter device installed in operative proximity with the surface to be observed.
In the autumn and spring a thin layer of water spread over the surface of an asphalt road or paved runway of an airfield can unexpectedly freeze. This makes the road or runway slippery and hazardous for traffic. This phenomenon is called "the black ice problem", since it is often very difficult to tell whether the black surface of an asphalt road is frozen or is only covered by a thin layer of water. Attempts have been made to observe the formation of "black ice" by, for example, temperature detectors, which are installed under the surface of the asphalt, and conductivity gauges. These methods have not been successful due to several drawbacks. The road surface might, for example, be treated with salt, which makes the measurement of temperature and conductivity unreliable.